1. Field of the Invention
The present invention relates generally to light projection devices, and more specifically to such a device suitable for use on aircraft instrument panels.
2. Description of the Prior Art
Modern aircraft instrument panels have a large number of gauges and other instruments for providing information to a pilot. These gauges must be illuminated at night. Nightime illumination is usually accomplished by mounting a light source in a housing adjacent each gauge. The optical design of the housing causes light to be projected onto the gauges of the instrument panel.
Such a technique has a problem with uniform illumination of instruments since the light source is located at one edge of the gauge. The far edge of a gauge tends to receive less light than the edge nearest the source. This variation in light intensity across the gauge makes it more difficult to read and does not meet adequately the needs of aircraft pilots.
It is often important to minimize the amount of light projected onto portions of the instrument panel other than the gauges to which it is directed. The amount of light projected directly onto the pilot's field of vision should also be minimized. Illumination control of each individual gauge is desirable. Glare from the aircraft instrument panel may cause night blindness or interfere with the performance of night vision goggles such as used on military aircraft. Minimizing the amount of light used on the instrument panel helps minimize degradation in the pilot's night vision caused by looking at the instrument panel. In order to minimize the amount of light applied, it is important that the light which is available is confined as much as possible to the gauges themselves, and that uniform illumination over the gauge surface is provided.
FIG. 1 illustrates diagrammatically how illumination of aircraft instrument gauges is accomplished. A planar instrument gauge 10 is typically 3 inches in diameter and mounted nearly flush with the surface of an instrument panel. A light source 12 is mounted on the instrument panel adjacent the gauge 10. Light source 12 provides, in most cases, the sole source of illumination of gauge 10.
In order to minimize the amount of light scattered from portions of the instrument panel other than the gauge 10, military specifications provide limits on the amount of light spillover which may occur beyond the edges of a 5 inch diameter circle 14 concentric with the gauge 10. For military helicopters illuminated with post lights, military specifications require that the illumination of every point of the surface of the gauge 10 be between 0.5 and 1.5 foot lamberts. Such specifications further require that the illumination caused by source 12 beyond the limits of the circle 14 be no greater than 0.1 foot lambert.
FIG. 2 is a side view of the gauge 10 and light source 12 of FIG. 1. As can be seen in FIG. 2, the gauge projects slightly above the surface of the instrument panel. Housing 12 contains a light source (not shown) internally, and light is projected from an opening 16 facing the gauge 10. Optics contained within the source 12, typically a prism, with no magnification, cause light projected from the opening 16 to be directed downward onto the gauge 10. Ideally, the projected light will be contained only within a cone delimited by lines 18 and 20. In practice, a reasonably large percentage, but not all, of the projected light occurs in this region.
Since one edge of the gauge 10 is more or less immediately adjacent the source 12, this portion of the gauge 10 tends to receive a significantly higher level of illumination than the opposite side of the gauge 10. The illumination falling on gauge 10 from source 12, is typically proportional to the inverse of the square of the distance from source 12. This uneven illumination over the surface of the gauge 10 is a problem which has not been satisfactorily solved using current light source 12.
It would be desirable for a light source to provide more uniform illumination of an instrument gauge, with a minimum amount of light spill-over beyond the boundary of the gauge. It would be especially desirable for such a source to be inexpensive and for it to be a plug compatible replacement for existing light sources.